Estrogens are involved in a variety of physiological processes, including regulation of arousal. Our studies indicate that estrogens increase three separate components of arousal;running wheel and home cage motor activity, sensory responsiveness (predominantly to tactile stimulation) and emotionality (fear conditioning) are all increased in estradiol-treated animals as compared to oil-treated animals. Microarray studies revealed that lipocalin-type prostaglandin D synthase (L-PGDS) is specifically altered in select brain regions of estrogen-treated mice. L-PDGS catalyses the formation of prostaglandin D2 (PGD2). PGD2 is an endogenous somnogen that induces c-FOS expression on sleep-active neurons in the ventrolateral preoptic area (VLPO). The sleep-promoting effects of PGD2 are mediated by increased A2A receptor binding in VLPO neurons. A2A agonists can directly excite a subpopulation of VLPO neurons. Taken together, we hypothesize that PGD2 and adenosine are key molecules involved in the signaling cascade by which estrogens increase arousal. Specifically, we will test whether the arousal-promoting effects of estrogen are mediated via a suppression of A2A signaling in the VLPO. The proposed experiments will provide critical insight into the signaling cascade underlying estrogen- mediated increases in arousal, and possibly reveal new therapeutic avenues in the treatment of disorders of vigilance and arousal.